This invention relates generally to optical power monitoring. In particular, the invention relates specifically to an optical bench apparatus having integrated monitor photodetectors and a method for monitoring optical power using the optical bench apparatus for optical power monitoring in optical modules. Monitoring the level of optical power emitted by transmitters may be a highly desirable feature in many optical modules, especially optical transceivers. Monitoring the optical power may allow the bias and/or modulation currents to be optimized to achieve the desired operating characteristics of both the transmitter itself, as well as the entire optical link. Such optimization of the bias and/or modulation currents may allow operating characteristics to be adjusted for temperature variations and/or degradation due to aging, optical alignment and/or other environmental factors. Monitoring the transmitted power and/or received power in an optical link may enable health monitoring and/or functions to be implemented in the transceiver. Further, monitoring the transmitted power and/or received power in an optical link may also enable built-in test functions to be implemented in the transceiver.
Optical power monitoring of transmitters has been implemented by various methods. One such method of optical power monitoring may create a back-reflection in the optical package and utilize this back-reflection to monitor the optical power. For example, U.S. Pat. No. 5,757,836 discloses TO-can-based transmitter optical subassemblies (TOSAs) that may implement such a method of optical power monitoring. A cap on the TO-can may provide a small back-reflection that may then be detected by a monitor photodetector placed either next to the transmitting devices and/or underneath the transmitting devices such that the cap of the TO-can may extend laterally beyond the extent of the transmitter die. The TO-can-based method has been acceptably implemented but may be best-suited to modules with a small number of transmitters. Further, the method may not scale well to parallel modules with a small form factor.
Another method and/or approach to optical power monitoring may utilize back-side emission of a transmitting device. Although such a method may be possible with a Vertical-Cavity Surface-Emitting Laser (VCSEL), implementation with an in-plane laser in which emission from the back facet may be exploited to monitor the power emission from the laser may be more feasible and/or effective. An example of this method and/or approach may be implemented with a 622 Mb/s Logic Interface DFB Laser Transmitter manufactured by Hewlett Packard. The technical specifications and other information for such a device may be found on the Internet in http://www.datasheetcatalog.org/datasheet/hp/XMT5170B-622-AP.pdf. This method and/or approach to optical power monitoring may be implemented in many different ways. For example, such a method of optical power monitoring may be monolithically integrated and/or heterogeneously integrated. A fundamental approach in such methods and/or approaches may use a transmission out of the back facet. The power of the back facet transmission may be a known ratio of the power out of the front facet. The front facet power may be used for the transmitter in the optical module, while the back facet power may be absorbed by a monitor photodetector to provide the power monitoring.
Various substrates have also been utilized as optical benches for integrating transmitters and/or photodetectors. Some of these optical benches have included backside microlenses formed by a variety of techniques. For example, such microlenses are disclosed in “Parallel Free-Space Optical Interconnects Based on Arrays of Vertical-Cavity Lasers and Detectors with Monolithic Microlenses,” Eva M. Strzelecka, et al., volume 37, issue 14, pp. 2811-2821, 1998.